Note: Descriptions are shown in the official language in which they were submitted.
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WO 02/24779 PCT/EP01/10516
Agueous coatings which dry at low temueratures
The invention relates to aqueous binder combinations which can be processed as
one-component systems to produce a coating which dries at below 100°C,
is
sandable and at the same time is stonechip-resistant and weathering-stable,
preferably in automotive OEM finishing, and to the preparation and use
thereof.
Established in automotive OEM finishing is a multicoat topcoat construction
which
is increasingly being realized using aqueous coating compositions as well. In
the case
of such modern coatings, requirements are also imposed on the economics. The
aim
is to apply the coatings in as few and as cost-effective individual steps as
possible.
In the standard coating structure in automotive OEM finishing, a metal surface
primed by cathodic electrocoating is coated first with a stonechip prevention
coat and
1 S a surfacer coat or with a combination of the two ("stonechip prevention
surfacer").
Atop these coats there is then applied a basecoat material, followed by a
clearcoat
material or, alternatively, a pigmented topcoat material.
The stonechip protection coat and/or stonechip protection surfacer coat
provides
compensation for unevennesses in the surface and, as a result of high
elasticity and
deformability, produces good resistance to stonechipping. For this coat,
therefore,
use has to date been made of soft polyester- or polyether-polyurethanes and
also
polyisocyanate or melamine crosslinkers. Prior to the application of basecoat
and
clearcoat or topcoat material, the stonechip protection surfacer is baked.
This is
necessary in order to improve the topcoat holdout and to abrade the surfacer
coat
before the upper paint coats are applied. After the clearcoat and/or topcoat
material
has been applied, baking is then carried out again. A disadvantage of this
process is
that it necessitates two expensive baking operations with temperatures >
120°C.
Coating materials which dry at lower temperatures, such as those which can be
formulated, for example, from polyacrylates, do not have the requisite
stonechip
resistance, since they crosslink to form brittle films.
~J ~ 3~ 'SEC
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The present invention was based on the object of finding a storage-stable
coating
formulation which can be processed as a one-component system and which in
addition to good stonechip resistance ensures good sandability and surface
hardness.
The coating ought additionally to be lightfast, so as to produce a weathering-
stable
coating even in the case of moderately hiding topcoat or basecoat materials,
or at
points where a pigmented topcoat or basecoat material is omitted entirely. An
essential requirement in addition is that the coating adhere to a variety of
substrates,
such as the plastics commonly used in automobile construction (pretreated
polypropylene or polyethylene, pretreated polypropylene/EPDM blends,
polyurethane RIM, ABS, polybutylene terephthalate, polycarbonate, polyamide,
SMC, BMC, et cetera, and also blends of said plastics) and also ferrous and
nonferrous metals and cured coating films based on different binders. Bearing
in
mind the low heat distortion resistance of the thermoplastics commonly used in
automobile construction (see above), it must be possible to cure/crosslink the
coatings at temperatures below 100°C.
One proposed solution for stonechip-resistant coatings and/or surfaces coats
based on
aqueous binders is described, for example, in EP-A 0 330 139. It is claimed
that a
polyacrylate can be admixed to an OH- and COOH-functional polyester. Exemplary
embodiments to that effect, however, are absent. Nor is there any description
of graft
copolymerization of the polyacrylate onto a polyester grafting base. The
claimed
dispersions of acid-functional polyesters are known to be of poor stability on
storage,
being subject to rapid chemical degradation through cleavage of ester bonds.
(e.g.
Jones, T.E.; McCarthy, J.M.; J. Coatings Technol. 76 (844), p. 57 (1995)).
EP-A 0 024 727, for example, describes a stoving enamel based on a combination
of
epoxy resin phosphoric ester, a water-dilutable polyester, and water-soluble
melamine resins. DE-A 4 000 748 describes, in working examples, aqueous
automotive surfacers based on water-dilutable hydroxy-functional polyester-
polyurethane resins, optionally further binders, and water-soluble amino
resins.
The stringent requirements imposed in particular by the automobile industry on
surfaces coatings are not yet fully met by these coatings. One improvement has
been
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achieved by further-developed, polyisocyanate-crosslinked surfacer coatings
(M. Bock, H. Casselmann, H. Blum "Progress in Development of Waterborne PUR-
Primers for the Automotive Industry", Proc. Waterborne, Higher Solids and
Powder
Coating Symp. New Orleans 1994). All of the systems referred to, however, have
the
disadvantage that the water-dilutable polyester or polyester-polyurethane
resins
employed are of limited storage stability when they are crosslinked with
reactive
amino resins and/or polyisocyanate resins for lower baking temperatures.
EP-A 0 980 880 describes stonechip-resistant, physically drying coatings
produced
using polyacrylate dispersions in combination with polyurethane dispersions.
The
sandability of these coatings, however, is inadequate.
WO PCT/00/01205 describes stonechip-resistant physically drying coatings
produced using polyacrylate dispersions. These combinations, however, exhibit
weaknesses in stonechip resistance at temperatures below 0°C.
The present invention was therefore also based on the object of providing
storage-
stable, aqueous, low-cosolvent, one-component, lightfast coating systems which
exhibit improved sandability and good stonechip resistance at baking
temperatures
< 100°C. In addition the coating system ought to offer adhesion to a
variety of
substrates used in automobile construction.
It has surprisingly been found that this object can be achieved by using a
combination of specific hard and highly elastic aqueous polyurethane
dispersions
which contain virtually no free OH groups, and highly reactive water-dilutable
melamine resins. Distinguishing between hard and soft dispersions can be done
by
way of the pendulum hardness of the films obtained by physical drying. The
hard PU
dispersion A is characterized by a Konig pendulum hardness (DIN 53157) of > 90
s,
whereas the soft PU dispersion B is characterized by a pendulum hardness of <
90 s.
In the coating formulation, which comprises additivization and pigmentation
customary for automotive surfacers, then, the binder fractions are combined as
follows:
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Of dispersion A, 20-90% by weight, but preferably 40-70% by weight, are used.
Of
dispersion B, 10-80%, preferably 30-60% by weight, are added. Furthermore, for
crosslinking, a reactive melamine resin is added in a fraction of 0-30% by
weight,
preferably between 5 and 20% by weight. The fractions of the 3 resin
components
add up to 100% binder fraction.
Particularly suitable for such combinations are polyurethane dispersions in
analogy
to WO PCT/00/01205, as a highly elastic component, and polyurethane
dispersions
in analogy to EP-A 0 269 972, as a hard component. The cited patent literature
describes the preparation of the polyurethane dispersions in what is called
the
acetone process. Dispersions of analogous compositions prepared by what is
called
the melt dispersion process are likewise suitable for the application. The
dispersions
are preferably neutralized using tertiary amines such as, for example,
triethylamine,
diisopropylethylamine, morpholine. These binders are optionally combined with
water-dilutable amino resins whose reactivity is adapted to the baking
temperature
(e.g., Cymel~ 327, 328 - Cytec Industries B.V., Rotterdam, Netherlands,
Maprenal~
VMF 3921 W - Vianova Resins GmbH and Co. KG, Frankfurt, DE).
Examples
Example 1
Water-dilutable, OH-containing, fatty-acid-modified polyester-polyurethane
dispersion in combination with reactive melamine resin (Bayhydrol~ FT 145
(Bayer
AG, Leverkusen, DE)/Cymel~ 328 (Cytec Industries B.V., Rotterdam, Netherlands)
35.8 g of a pigment grinding paste set out below are stirred together with
50.8 g of a
45%, fatty-acid-modified polyester-polyurethane dispersion (Bayhydrol~ FT 145)
and 3.0 g of a 85%, commercial aqueous melamine resin (Cymel~ 328) and the
system is diluted with 10.4 g of distilled water to a spray viscosity of
approximately
17 s ISO cup 5 mm.
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Example 2
Water-dilutable, low-OH-group, hard polyurethane dispersion (Bayhydrol~ PR
135,
Bayer AG, Leverkusen, DE) in combination with an elastic polyurethane
dispersion
in accordance with EP-A 0 330 139.
This elastic polyurethane dispersion consists of 272 g of an aliphatic
polycarbonate-
diol (Desmophen 2020~, Bayer AG, Leverkusen, DE, molar weight 2000), 272 g of
a polyesterdiol based on adipic acid, hexanediol, neopentyl glycol (molar
weight
1700) and 26.8 g of dimethylolpropionic acid, weighed in under nitrogen
atmosphere
and heated to 65°C. Subsequently 11.3 g of trimethylolpropane, 250 g of
acetone,
106.6 g of isophorone diisocyanate, 75.9 g of hexamethylene diisocyanate and
0.025% of dibutyltin dilaurate are added and the mixture is heated at reflux
1 S temperature until the NCO value reaches the theoretical level or slightly
below. After
cooling to 45°C, 17.2 g of triethylamine (degree of neutralization 85%)
and 1150 g
of distilled water are added and stirring is continued at from 40 to
50°C until free
NCO groups are no longer detectable. The acetone is subsequently removed by
distillation. The resultant, very fine, elastic polyurethane dispersion has a
solids
content of approximately 40% with a pH of approximately 7.7 and a viscosity of
approximately 7800 mPa.s/23°C.
34.5 g of the pigment grinding paste as used in Example 1 are mixed with 34.9
g of a
35% polyurethane dispersion (Bayhydrol~ PR 135) and 30.6 g of the above-
described 40%, elastic polyurethane dispersion in accordance with EP-A 0 330
139,
using a stirrer.
Example 3
Water-dilutable, low-OH-group, hard polyurethane dispersion in combination
with
reactive melamine resin (Bayhydrol~ PR 135/Cymel~ 328)
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34.4 g of the pigment grinding paste as used in Example 1 are stirred together
with
62.7 g of a 35% polyurethane dispersion (Bayhydrol~ PR 135) and 2.9 g of an
85%,
commercial aqueous melamine resin (Cymel~ 328).
S Example 4
Water-dilutable, low-OH-group, elastic polyurethane dispersion in accordance
with
EP-A 0 330 139 in combination with reactive melamine resin (Cymel~ 328)
37.3 g of the abovementioned pigment grinding paste are stirred together with
59.5 g
of a 40% polyurethane dispersion in accordance with EP-A 0 330 139 (see
Example 2 above) and 3.1 g of an 85%, commercial aqueous melamine resin
(Cymel~ 328).
Example 5
Water-dilutable, low-OH-group, hard dispersion (Bayhydrol~ PR 135) and elastic
polyurethane dispersion in accordance with EP-A 0 330 139, in a 1:1 ratio, in
combination with reactive melamine resin (Cymel~ 328)
35.8 g of the pigment grinding paste as used in Example 1 are intimately mixed
with
32.6 g of a 35% polyurethane dispersion (Bayhydrol~ PR 135), 28.6 g of a 40%,
elastic polyurethane dispersion in accordance with EP-A 0 330 139 (see Example
2
above) and 3.0 g of an 85%, commercial aqueous melamine resin (Cymel~ 328),
using a stirrer.
Pigment grinding paste for paint Examines 1-5
A predispersed slurry consisting of 10.8 g of 70%, water-dilutable polyester
resin
(Bayhydrol~ D 270), 21.1 g of distilled water, 1.5 g of 10% strength
dimethylethanolamine in water and 2.8 g of commercial wetting agents, 27.7 g
of
titanium dioxide (Tronox~ R-FD-I, Kerr McGee Pigments GmbH and Co. KG,
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Krefeld, DE), 0.3 g of black iron oxide (Bayferrox~ 303 T Bayer Ag,
Leverkusen,
DE)), 27.9 g of barium sulfate (Blanc fixe Micro, Sachtleben Chemie GmbH,
Duisburg, DE), 6.8 g of talc (Micro Talc IT Extra, Norwegian Talc, Frankfurt,
DE)
and 1.0 g of anti-settling agent (Aerosil~ R 972, Degussa-Hiils AG, Frankfurt,
DE)
is ground to a paste in a commercial bead mill, with cooling, for 30 minutes.
This results in paint systems having a binder pigment/filler ratio of
approximately
1:1 ) and binder combinations below, solids contents in parts by weight (pbw),
and
efflux times in the ISO 5 mm cup immediately and after storage at 40°
for 7 d.
Paint example 1 2 3 4 5
Binder - resin solids (pbw)
OH polyester 90.0 - - - -
PUD, hard, low-OH - 50.0 90.0 - 45.0
PUD, elastic, low-OH - 50.0 - 90.0 45.0
Melamine 10.0 - 10.0 10.0 10.0
100.0 100.0 100.0 100.0 100.0
Solids content 50.9% 49.1% 48.9% 53.1% 50.9%
Efflux time ISO 5 17 s 12 s 15 s 13 12 s
mm s
Efflux time after 24 s 12 s 13 s 13 12 s
7 d at 40C s
These paints are applied to the substrates below using a gravity-feed cup-type
spray
gun with a nozzle diameter/1.5 mm and an atomizer pressure of 5 bar in a
resulting
dry film thickness of 25-35 Vim. The wet paint films are flashed off at
23°C for 5
minutes and then baked in a forced-air oven. The substrates are glass plates
in the
case of the pendulum hardness and gloss tests, degreased steel panels in the
case of
adhesion/cross-cut, Erichsen cupping, and sandability, and cathodically
electrocoated
steel panels used in automobile production in the case of the stonechip tests.
The test results obtained are as follows:
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Paint example ~ 1 ~ 2 3 ~ 4 5
Baking conditions surfacer: 10 min at RT and 30 min at 90°C
Pendulum hardness 21 s 54 s 108 s 30 65 s
s
Gloss 60 93% 51% S1% 56% 53%
Erichsen cupping 10 mm 10 mm 10 mm 10 10 mm
mm
Cross-cut (0-5) 0 1 0 0 0
Sandability, wet (1-4)3 2-3 1 2-3 1-2
Stonechip tests
System: cathodically electrocoated metal panel, surfacer (10 min at RT and 30
min at
S 90°C), aqueous basecoat "black metallic" (15 ~,m, 10 min at
80°C), 2K acrylic
clearcoat (40 Vim, 30 min at 90°C)
A) VDA* bombardment, 2 times S00 g; 1.5 bar
Intercoat adhesion
electrocoat/surfacer 2 2-3 2 1 1
(1-3)
Intercoat adhesion
surfacer/topcoat (1-3)1 1 1 1 1
Stonechip result (1-10)1 2 2-3 ~ 1 ~ 1
[*VDA is the German automobile manufacturers association]
B) BMW wedge at RT and -30°C
Flaking from metal
panel
RT/-30C 0/1 mm 0/0 0/2 mm O/0 0/0
Pendulum hardness: Konig oscillation test, DIN 53 157
Gloss measurement, 60°, in accordance with DIN EN ISO 2813
Erichsen cupping in accordance with DIN EN ISO 1520
Cross-cut in accordance with DIN EN ISO 2409
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Sandability in accordance with VW specification, wet, by hand, using abrasive
paper
with standardized grain size 320-400, index 1-4 (1 = complete removal of the
sanding dust from the abrasive paper by water jet, 4 = not sandable)
Stonechip test VDA in accordance with VW specification with bombardment with 2
times 500 g of steel shot at an air pressure of 1.5 bar. Index 1-10 (1 = no
penetrations, 10 = very large and numerous instances of flaking from the metal
panel)
Intercoat adhesion, index 1-3 (1 = good, 3 = no adhesion)
Stonechip test in accordance with BMW specification with single chipping
tester
ESP 10 from Byk at test pressure of 3 bar. Determination of the parting plane
in mm
from the steel panel.
The results show that in Example l, in accordance with expectations, the
presence of
OH-group-containing polyester dispersion and highly reactive melamine resin
does
not ensure sufficient storage stability. More favorable is the behavior of the
stability
with melamine resin in Examples 3-5, when low-OH-group polyurethane
dispersions
are used. The desired hardness of the paint film can be set by the fraction of
the low-
OH-group, hard and soft dispersions in the formulation. A good result in terms
of
stability, sandability, and impact resistance is shown by the inventive
Example 5.
Without melamine resin, these mixtures are likewise stable and suitable for
use.
